Add note to _vendor/README.rst about pkg_resources patch.

Author: domdfcoding

src/pip/_vendor/README.rst

@@ -100,7 +100,8 @@ Modifications
 
 * ``setuptools`` is completely stripped to only keep ``pkg_resources``.
 * ``pkg_resources`` has been modified to import its dependencies from
-  ``pip._vendor``.
+  ``pip._vendor``, and to use the vendored copy of ``platformdirs``
+  rather than ``appdirs`.
 * ``packaging`` has been modified to import its dependencies from
   ``pip._vendor``.
 * ``html5lib`` has been modified to import six from ``pip._vendor``, to prefer